In order to approve a tentative $400 million package for Pakistan, Congress and the administration must waive a cutoff of aid imposed only last April. This was required by strong evidence that Pakistan is developing nuclear weapons.

In turning a blind eye to Pakistani President Zia ul-Haq's nuclear plans, the US also is under pressure to show evenhandedness by permitting export of nuclear fuel to India. Under the 1978 Nuclear Non-Proliferation Act, such shipments are supposed to end after March 10 because India has refused to permit international inspection of all its nuclear facilities, including the plants that produced the test bomb it exploded in May 1974.

No final action on the Pakistan aid package is likely until after General Zia has reviewed the proposals that Pakistani Foreign Minister Agha Shahi discussed here with President Carter and other top officials. It reportedly excludes advanced combat planes so as not to antagonize Indian Prime Minister Indira Gandhi's new government, which has refused to criticize the Soviet invasion of Afghanistan.

The package does include a reported $200 million in antitank and antiaircraft weapons, light arms, artillery, and tanks for defense by Pakistan's 400,000-man armed forces of its exposed northwest frontier. Another $200 million in economic assistance may round out the package.

Administration officials expected Defense Secretary Harold Brown to return here from his Jan. 9-13 visit to China with some details on what he has termed "parallel" Chinese aid to Pakistan, and possible Chinese help to the anti-Soviet Afghan Muslim rebels based in Pakistan.

When China and Pakistan formally opened their all-weather, high-altitude truck highway across the Karakoram Mountains in June 1978, India bitterly protested, calling it a strategic road for military movements. But most of the past Chinese military supplies to Pakistan, including light arms, fast missile and hydrofoil boats, and a few of China's Shenyang F-4 aircraft have arrived by sea, US analysts say.

Assistant US Secretary of State Warren Christopher and other senior US officials flew to Europe Jan. 13 to discuss what Secretary Brown has called a "consortium" of Western nations for military aid to Pakistan.

Since the 1960s, the Pakistanis have obtained -- through imports or licensing agreements -- several submarines from France; helicopters from France and Britain; Saab trainer and ground-attack aircraft from Sweden; Cobra antitank missiles from West Germany; and possibly as many as 400 AMX- 13 French-designed tanks from Argentina.

The US embargoed arms sales to Pakistan and India after the outbreak of the Indo- Pakistan war of 1965. The embargo was eased, then lifted in 1975 to permit case-by- case cash sales. These averaged less than $100 million annually until last April, when Pakistan nuclear development was publicized by the US and all aid except that in the pipeline and some training was cut.

The 1977 Symington admendment bars US aid other than cash sales to nations not renouncing nuclear weapons. Rep. Clement Zablocki (D) of Wisconsin, chairman of the House Foreign Affairs Committee, and other congressmen have been drafting a special law to exempt Pakistan.

After India and Pakistan, Argentina has now arrived at the threshold of entry into the nuclear weapons club, administration analysts say.

President Carter's seemingly unsuccessful policy of preventing nuclear proliferation will be futher tested next summer during a conference to review the 1968 nonproliferation treaty (NPT). Like Israel and South Africa, which also are "threshold" nations, Pakistan, India, and Argentina have ignored the NPT.